Mice were exposed to approximately 20 ppm of nitrogen dioxide (N02 or 15-N02) for 5-6 hours, or 1 g/kg of body weight of morpholine by gavage, or both. Treatments were repeated daily for 5 consecutive days. N-nitrosomorpholine (NMOR) was found in whole carcasses (16 to 146 ng/mouse) in all animals that had been exposed to both N02 and to morpholine, but not in tissues from animals that had been exposed to either chemical alone. Approximately one-third of the NMOR was found in the gastrointestinal tract. The coadministration of 2g/kg of sodium ascorbate or 1 g/kg of alpha-tocopheryl acetate had no effect on the amount of NMOR that was found in any tissue. Approximately one-third of the total amount of NMOR that was found in the body was found in the stomach. N-nitromorpholine was not detected in any tissue. The results of the 15-N02 experiments are not yet available. We concluded that the repeated, concurrent exposure of mice to N02 by inhalation and to morpholine by gavage resulted in the in vivo formation of detectable quantities of NMOR. No tumors were seen in related 2-year studies from which we conclude that concurrent exposure to N02 and nitrosatable amines probably does not constitute a carcinogenic hazard in laboratory animals.